Communication of data is a necessary adjunct of modem society. Data is communicated to effectuate many different types of communication services. Data communication systems are provided through which to effectuate the communication of the communication service.
A communication system is formed, at a minimum, of a sending station and a receiving station, connected theretogether by way of a communication channel. Data that is to be communicated by the sending station is converted, if necessary, at the sending station into a form to permit its communication upon the communication channel. The receiving station operates to detect the data communicated upon the communication channel. Once detected at the receiving station, the informational content of the data is recovered.
Many different types of communication systems have been developed and implemented. Different types of communication systems are utilized to effectuate different types of communication services. And, as advancements in communication technologies permit, improvements to existing communication systems are made, and new types of communication systems are developed. The advancements, generally, permit data to be communicated at increased throughput rates and in manners to increase the likelihood that the informational content of the data can be recovered at a receiving station in spite of poor communication channel conditions.
A radio communication system is a type of communication system. In a radio communication system, communication channels are defined upon radio links formed between the sending and receiving stations of the communication system. Such communication channels, referred to as radio communication channels, do not require that a wireline connection interconnect the sending and receiving stations. Construction of the infrastructure of a radio communication system, as a result, is generally less costly than the corresponding construction costs of the infrastructure of a conventional, wireline communication system. And, a radio communication system can be implemented as a mobile communication system as communications are effectuated by way of radio channels rather than fixed, wireline channels.
Radio links, upon which the radio communication channels are defined, are formed upon a portion of the electromagnetic spectrum. The portion of the electromagnetic spectrum allocated for use by a particular radio communication system is typically band width-constrained. That is to say, the communication capacity of a radio communication system is limited by the amount of spectrum allocated to the communication system. The only manner by which to increase the communication capacity of such a system is to increase the efficiency by which to utilize the spectrum allocated to the communication system.
Digital communication techniques permit increased efficiency of usage of the spectrum allocated to the communication system. First use of such techniques, therefore, in a communication capacity of a radio system generally can be increased.
Typically, digital communication techniques involve the digitizing of the data that is to be communicated. When the data is digitized, the data becomes formed of digital bits. The digitized bits are sometimes formatted into sequences according to a formatting scheme to form packets or frames of data.
The data, once formatted into the packets or frame, are communicated during discrete intervals upon a communication channel. Once delivered to a receiving station, the packets or frames are concatenated together, and the informational content of the data is recovered. Because the data can be communicated at discrete intervals, a circuit-switched connection need not be maintained between receiving communication station. Instead, packet-switched channels can be used in which two, or more, sending and receiving station pairs can share the same radio link or channel.
Conventional local area networks (LANs) communicate packets of data to effectuate communications between sending and receiving stations defined therein. Wireless networks, operable in manners analogous to LANs, referred to as WLANs (wireless local area networks) have also been developed and also are utilized to communicate data upon radio channels defined upon radio links.
A variant of an IEEE (Institute of Electrical and Electronic Engineers) 802.11 operating specification sets forth a wireless operating protocol that has been proposed as the operating standard for a WLAN. And, the operating specification has been proposed as the operating protocol by which to operate a so-called 4th generation (4G) cellular communication system. The operating specification of the IEEE 802.11 standard sets forth an operating protocol for multi-user communications.
At least one implementation of a WLAN anticipates, or otherwise uses, OFDM (orthogonal frequency division multiplexing) techniques. OFDM techniques effectively form a hybrid of a multi-carrier modulation (MCM) and frequency shift keying (FSK) modulation. Frequency-divided carrier frequencies are defined in an OFDM system. And, the carriers are selected to be orthogonal to one another, such as by separating the carriers by integer multiples of the inverses of symbol duration of parallel bit streams that are to be applied thereto. The orthogonal carriers are transmitted simultaneously, thereby permitting an entire allocated channel to be occupied through an aggregated sum of narrow, orthogonal sub-bands.
Conventionally, OFDM techniques perform transformations between time-domain data and frequency-domain data through the use of a unitary transformation, namely the discrete Fourier transform (DFT). And, through use of such a DFT together in conjunction with the use of cyclic prefixes permits a receiving station to be of relatively simple construction in the presence of frequency-selective channel conditions.
A DFT is, however, a complex transform. And, data that is transmitted in a communication system that utilizes OFDM techniques must be transmitted in quadrature. Single side band transmissions, for example, cannot be transmitted in a DFT-based OFDM system. Also, through the use of the DFT as a sole unitary transformation used in OFDM-based systems, the use of other transformation-types that might be more appropriate for particular communication channel conditions are, in existing systems, unavailable.
While use of non-DFT, unitary transformations have been considered for use in various communication systems, the use of such transformations have not been considered in a manner that would limit the complexity required of equalization operations at a receiving station operable in such a system.
If a manner could be provided by which to utilize non-DFT transformations in an OFDM communication system, improved communication performance would be possible.
It is in light of this background information related to communication systems that utilize OFDM techniques that the significant improvements of the present invention have evolved.